Sulphide mineralogy, metamorphism and deformation in the Matchless massive sulphide deposit, Namibia

Abstract

The sulphide mineralogy of the Matchless massive sulphide deposit has been reviewed and the morphology, mineral chemistry and paragenesis of the opaque minerals described and related to genetic processes. Amphibolite facies metamorphism at 510–540 °C and 6–6.5 kbar and accompanying polyphase penetrative deformation of the ores has resulted in prograde corrosion and replacement of pyrite, onto which is superimposed retrograde overgrowths and recrystallisation and annealment of porphyroblastic pyrite. Pyrite exhibits examples of both brittle and ductile deformation, although late retrograde cataclasis of recrystallised pyrite is the dominant preserved texture. Contrasts in competence between pyrite, the more ductile sulphides and silicates has resulted in the most extreme example of deformation: the ‘breccia ores’, in which clasts of silicate minerals have been enclosed in a matrix of chalcopyrite and pyrrhotite. The FeS content of sphalerite (spFeS) depends, to a large extent on the coexisting iron sulphides (spFeS: 7.22 ± 1.22% in samples where pyrite is dominant, 16.78 ± 0.93% where pyrrhotite is dominant). Only where the sphalerite has been buffered by pyrite and pyrrhotite is application of the sphalerite geobarometer possible. In such cases, the sphalerite compositions have apparently not re-equilibrated during retrograde metamorphism. In four samples, spFeS values of 10.5–13.5% give pressure estimates of 6–8 kbar. Such estimates are consistent with the tectonic history of the region.